Introduction

In the last session, we looked at the idea of the celestial sphere, and how the Earth's rotation means that stars appear to follow nightly paths on the celestial sphere. Our model of the celestial sphere is geocentric, meaning that the Earth is taken to be at the centre of the sphere. Of course, in reality the Earth orbits around the Sun. The Earth's orbit around the Sun will cause the apparent position of the Sun to move around the celestial sphere over the course of a year. In this session, we'll look at that in detail, and examine the consequences for the observability of stars.

The figures above show the motion of the Earth around the Sun. Obviously, they are not to scale! The effect of the Earth's motion is to make the Sun appear to be on one side of the celestial sphere in December, and on the opposite side in June. This means that, over the course of a year, the Earth's orbit around the Sun means that the Sun's position of the celestial sphere changes slowly.

What does this look like over the course of a single night? Over one night, the Earth has moved a small amount around its orbit, and so it is a reasonable approximation to treat the Sun as a fixed point on the celestial sphere. Therefore over a single night, we can say that the Sun moves through the sky like any other star on the celestial sphere.

Generally speaking, we can only see stars once the Sun has set. It follows that the stars which are visible at any time of year are those which are opposite the Sun. As a result, the night sky in December looks very different to the night sky in June. As astronomers, we need to make detailed calculations to work out if a given star is visible at a certain type of year. To do that, we need to look in more detail at the path the Sun follows on the celestial sphere over the course of a year.